Rotselaar and Wezemaal were first mentioned in written accounts in 1044. Only a century later, between 1138 and 1152, Werchter appears in historical documents.[3]

In the 12th century, Rotselaar and Wezemaal were ruled by the Duke of Brabant, whereas the Counts of Aarschot and the House of the Berthouts controlled Werchter. From about 1170, vassals of the Duke of Brabant settled at Wezemaal and Rotselaar. In the course of the 13th century, these vassals rose to the nobility and started to "rule" the fiefdoms of Rotselaar and Wezemaal as lords. In the 14th century, the Lords of Wezemaal and Rotselaar managed to detract Werchter (and Haacht) from the sphere of influence of the House of the Berthouts, thus uniting the three villages of Wezemaal, Werchter and Rotselaar for the first time in history, which from that moment on together formed the Land, or the Barony of Rotselaar. The Barony of Rotselaar passed into the hands of the powerful House of Croÿ in 1516, to be added to the Margraviate of Aarschot, which was later elevated to the Duchy of Aarschot in 1533.

Until deep in the 19th century, the inhabitants lived primarily of agriculture. From 1488 onwards, the population of Wezemaal, Werchter and Rotselaar was severely hit by periodically recurring wars (1488-1489, 1542, and a series of wars from 1570 up to 1750). From 1750, welfare began to increase again, first thanks to agricultural innovations and in the second half of the 19th century thanks to the effects of the Industrial Revolution.

An artillery duel was fought in Rotselaar in the First World War, known as the ‘Slag aan de Molen’ (English: Battle of the Mill). In that battle, 360 Belgian and German soldiers were killed. During the First World War, a total 67 houses were burned and 38 civilians were killed in Rotselaar.

The origin of the placenames is unclear. Rotselaar is thought to mean "laar of Hrosda". A laar (plural form is laren) is an open spot or clearing in a forest suitable for living, laren were used quite intensively by man in the past, amongst others for grazing the cattle, and Hrosda is a male Germanic name. Wezemaal is believed to come from "Wis" and "male" (a depression), and Werchter is thought to be a watername, but the meaning of the name is unknown.[3]

When the name of Rotselaar first appeared in written accounts, it was spelled "Rotslar". Over the centuries, this evolved into "Rotselaer", and eventually into "Rotselaar". In Middle Dutch, the "e" in "Rotselaer" was used to show that the vowel preceding it sounds longer, in modern spelling the vowel is doubled to achieve the same effect, which gives "Rotselaar".

The name "Rotselaar" can have four different meanings, and it is important to make a distinction between these meanings in order to avoid ambiguities. In a first meaning, "Rotselaar" refers to the whole of the municipality of Rotselaar as it exists since the merger of municipalities that came into effect on January 1, 1977 and reduced the number of autonomous municipalities in Belgium to 589. If used in this sense, the name "Rotselaar" includes Wezemaal and Werchter. The term "Groot-Rotselaar" (Greater Rotselaar) is also commonly used to refer to Rotselaar, Wezemaal and Werchter as a whole. In a second meaning, "Rotselaar" refers specifically to the town of Rotselaar proper as it existed before the merger with Wezemaal and Werchter in 1977. This is, especially within Groot-Rotselaar, the most common meaning of the name "Rotselaar". In a third meaning, "Rotselaar" refers to the historical Land of Rotselaar, which comprised not only modern-day Rotselaar, Wezemaal and Werchter, but also other villages, such as Haacht and Wakkerzeel. The name "Rotselaar" in this sense is rarely used.

Rotselaar can also have a fourth meaning, where it refers to Rotselaar-Centre, as opposed to Rotselaar Heikant, which is also referred to as simply "Heikant". Rotselaar proper (as it existed prior to 1977) consists of two parts: Rotselaar-Centre and Rotselaar Heikant. If used in this sense, the name "Rotselaar" refers only to Rotselaar-Centre and doesn't include Heikant. In most cases, the name "Rotselaar" includes Rotselaar Heikant, but the name "Heikant" is commonly used to distinguish between the two parts of Rotselaar.

The current Mayor of Rotselaar is Dirk Claes (CD&V). His primary responsibilities as mayor include, but are not limited to: security (police, fire service and mobility), personnel, administration and organisation, festivities, honorary and legal affairs, and tourism.[5] The mayor is assisted by a number of aldermen, who together form the College of Mayor and Aldermen. Rotselaar is entitled to five aldermen because it has between 10,000 and 19,999 inhabitants. The College of Mayor and Aldermen is also sometimes commonly referred to as the College of Aldermen.

The official coat of arms of the municipality of Rotselaar was adopted by the municipal council in 1968 and ratified by Royal Decree in 1973. After the merger with the municipalities of Werchter and Wezemaal, the coat of arms was confirmed by the municipal council and 1981 and ratified in 1982. The coat of arms consists of three fleur-de-lisgules on a field of silver.

In line with many other municipalities adopting modern fashions, the municipal government decided in 2002 to introduce a new house style with a modern logo replacing the coat of arms in communication and on municipal documents.[6]

There are 7 primary schools in Rotselaar.[8] There is also a secondary school in Rotselaar, the Montfortcollege, which has a good reputation in the field of education in the region. The Montfortcollege also offers boarding facilities.[9] While many of Rotselaar's youths attend primary schools in Rotselaar itself, most go to secondary schools outside of Rotselaar because the waiting lists for the Montfortcollege are often very long, except for boarding school students. Of the youths who attend secondary schools outside of Rotselaar, most go to school in Leuven. The "Hagelandse Academie voor Beeldende Kunst" (Hageland Academy for Visual Arts) is also located in Rotselaar.[10]

The keep "Ter Heide", this unique tower is based on a Greek cross floorplan and was probably built around the year 1350 on order of Gerard vander Heyden, Drossard of Brabant, who rose to the nobility through marriage. The keep, and the moat surrounding it, was meant to stress his prestige. The keep has six floors that are connected by a spiral staircase. The basement floor and the entrance level are roofed with a barrel vault. The tower was built in brick on a whitestone base, which is directly in the water. The construction was embellished with sandstone and ironstone. The keep has been protected as a monument since July 3, 1942 and was thoroughly restored in 1948. The keep "Ter Heide" is no longer open to the public.

The watermill "Van Doren", this watermill was first mentioned in the 12th century and was repeatedly expanded and restored since the 16th century. All these phases were preserved, so the building offers and overview of its archeological and industrial past. A new turbine has been placed in the mill, so nowadays it can generate electricity once more.

The Sint-Jan-De-Doperkerk (Saint John the Baptist's Church) in Werchter, this church dates back to 1439.

The Sint-Pieterskerk (Saint Peter's Church) in Rotselaar, a Neo-Gothic church in the centre of Rotselaar which was rebuilt in 1846.

The Domain Ter Heide is the main recreational domain in Rotselaar. It is also called, in Dutch, "het meer van Rotselaar" or "de Plas van Rotselaar" (the lake of Rotselaar). It is popular among walkers, cyclists and bird-watchers. Fishing is allowed in the fishing zone domain from June 1 to April 15, between sunrise and sunset. But you have to have a public Flemish fishing permit, which is available in the post office.[11]

A variety of other leisure activities are also possible, such as swimming or sunning. Outside of the swimming season, the domain's swimming zone is open daily from 08:30 to sunset, but during the swimming season it is open from 10:00 to 20:30. You have to pay for entry and swimming is only allowed when lifeguards are present and the green flag is out. The swimming season lasts roughly from the middle of May to the end of August, depending on the weather. The swimming zone is delimited by a ditch and a line of buoys, and swimming is prohibited outside of this zone, among other reasons because there is a surfing zone as well. For reasons of hygiene, dogs are not allowed during the swimming season in the entire domain, however, from September 1 dogs on a leash are allowed in the domain, but not in the swimming zone.

Windsurfing is allowed from the Easter holidays to October 15 in the surfing zone, which is separate from the swimming zone for reasons of safety. Ice skating on natural ice is possible as well, though rarely, but it is strictly forbidden to enter the ice unless the municipal government explicitly allows ice skating. If the municipal government allows ice skating, it will publish so in the local press, the "Dorpskrant" (a local newspaper published by the municipal government) and on the website of Rotselaar. There is also a nature zone in the domain, which is off-limits to the public. Other possible activities include, but are not limited to, squash and beach volleyball.

Rock Werchter is a music festival held annually during the first weekend of the summer holidays in Werchter. It was first organised in 1974 and since 2003 the festival lasts four days, and the 2003 and 2005 editions won the Arthur award for the best festival in the world of the International Live Music Conference. It's the largest music festival in Belgium and one of the largest festivals in Europe. It's even famous over the Belgian borders. Each year, many renowned groups and artists perform at Rock Werchter, and over 320,000 people come to the festival. Originally it was a double-festival, called "Rock Torhout-Werchter", with two festival areas on different places in Belgium: one in Werchter and one in Torhout. There is also a "Rock Werchterroute", a cycling route, around Werchter and Leuven, which also organises an annual music festival which is among the most popular in Belgium, Marktrock.

The request for cooperation came from Sal itself. Basilio Ramos, then Mayor of Sal, sought to establish a link with a municipality in the vicinity of Leuven. As a former student of the K.U.Leuven he wanted to strengthen the ties with Belgium. This form of cooperation is different from the others. The cooperation with Sal is a form of development cooperation where the accent is on exchange and partnership in the administrative and the professional fields, and in particular on the exchange of experiences in the fields of environment and youth work.

In 1989, the municipal government adopted the Romanian village of Vrânceni, which is a part of Căiuţi in the eastern county of Bacău, as part of the project "AdoptieDorpen Roemenië" (Adoption Villages Romania). The local action committee organises various activities throughout the year, of which the yields go to the adoption village. Among others, a school building was constructed in Vrânceni with the help of Rotselaar.[12]